The invention relates to a hydroformylation process.
It is known that aldehydes can be produced by reacting an olefinically unsaturated compound with carbon monoxide and hydrogen in the presence of a rhodium-organophosphite ligand complex catalyst, and that preferred processes involve continuous hydroformylation and recycling of the catalyst solution as is disclosed, for example, in U.S. Pat. Nos. 4,148,830; 4,717,775 and 4,769,498. Such aldehydes have a wide range of known utility and are useful, for example, as intermediates for hydrogenation to aliphatic alcohols, for aldol condensation to produce plasticizers, and for oxidation to produce aliphatic acids.
Notwithstanding the benefits of such rhodium-organophosphorous ligand complex catalyzed liquid recycle hydroformylation processes, stabilization of the catalyst and organophosphorous ligand is a primary concern. Loss of catalyst or catalytic activity due to undesirable reactions of the highly expensive rhodium catalysts are detrimental to the production of the desired aldehyde. Degradation of the organophosphorous ligand employed during the hydroformylation process can lead to the formation of detrimental species, such as poisoning organophosphorous compounds, inhibitors, or acidic by-products, that can lower the catalytic activity of the rhodium catalyst. Production costs of the aldehyde product increase when productivity of the catalyst decreases.
Hydrolytic instability of hydrolyzable organophosphite ligands is a major cause of ligand degradation and catalyst deactivation for rhodium-organophosphorous ligand complex catalyzed hydroformylation processes. All organophosphites are susceptible to hydrolysis to some degree, the rate of hydrolysis generally being dependent on the stereochemical nature of the organophosphite. Typically, the bulkier the steric environment around the phosphorus atom, the slower the hydrolysis rate. For example, tertiary triorganophosphites, such as triphenylphosphite, are more susceptible to hydrolysis than diorganophosphites, such as those disclosed in U.S. Pat. No. 4,737,588, and organopolyphosphites such as those disclosed in U.S. Pat. No. 4,748,261 and U.S. Pat. No. 4,769,498. All such hydrolysis reactions invariably produce phosphorus acidic compounds that catalyze the hydrolysis reactions. For example, the hydrolysis of a tertiary organophosphite produces a phosphonic acid diester, which is hydrolyzable to a phosphonic acid monoester, which in turn is hydrolyzable to H3PO3 (phosphorous acid). Moreover, hydrolysis of the ancillary products of side reactions, such as between a phosphonic acid diester and the aldehyde or between certain organophosphite ligands and an aldehyde, can lead to production of undesirable strong aldehyde acids, e.g., n-C3H7CH(OH)P(O)(OH)2.
Even highly desirable sterically-hindered organobisphosphites that are not very hydrolyzable can react with the aldehyde product to form poisoning organophosphites, e.g., organomonophosphites, which are catalytic inhibitors and which are far more susceptible to hydrolysis and the formation of such aldehyde acid by-products, e.g., hydroxy alkyl phosphonic acids, as shown, for example, in U.S. Pat. No. 5,288,918 and U.S. Pat. No. 5,364,950. Further, the hydrolysis of organophosphite ligands may be considered to be autocatalytic in view of the production of such phosphorus acidic compounds, e.g., H3PO3, aldehyde acids, such as hydroxy alkyl phosphonic acids, H3PO4 and the like, and if left unchecked the catalyst system of a continuous liquid recycle hydroformylation process will become more and more acidic over time. The eventual build-up of an unacceptable amount of phosphorus acidic materials can cause the total destruction of the organophosphite present, thereby rendering the hydroformylation catalyst totally ineffective (deactivated) and the valuable rhodium metal susceptible to loss, e.g., due to precipitation and/or deposition on the walls of the reactor. For example, in U.S. Pat. No. 5,741,944, a buffered extractor can be used to remove acidic species as they are formed, but this extraction is done outside of the reactor system and can be overwhelmed in some cases. The acid mitigation does not occur under the high temperature and multiple hour residence time conditions of the reactor, thus some degradation may occur before the acid neutralization can occur. Also, sodium-based oxy-acid buffers have shown a tendency to deposit Na-based solids (primarily of neutralized acidic species) that can cause severe operating difficulties, including plant shutdowns.
Numerous methods have been proposed to maintain catalyst and/or organophosphite ligand stability. For instance, U.S. Pat. No. 5,288,918 suggests adding to the reaction zone a catalytic activity enhancing additive, such as water and/or a weakly acidic compound; U.S. Pat. No. 5,364,950 suggests adding to the reaction zone an epoxide to stabilize the organophosphite ligand; and U.S. Pat. No. 5,741,944 teaches adding an oxyacid salt buffer to the extractor, optionally with amine additives, to remove acidic species from the catalyst solution. A further enhancement of the buffered extractor is taught in WO 2012/064586, wherein a water-washing step is added to remove metal salts derived from the oxyacid salt buffer prior to recycling the catalyst solution to the reaction zone.
U.S. Pat. No. 5,744,649 teaches extraction and removal of the acidic species using unbuffered water, i.e., a “water-only extractor.” However, maintaining the desired effective pH of the catalyst solution requires a very large flow of de-ionized water, which results in elevated product, ligand and catalyst loss due to entrainment or dissolution in the water phase. Amine additives optionally may be used for the purpose of rejuvenating deactivated catalyst or for preventing acidic impurities from complexing the active catalyst. The amines may also act to “deliver” the neutralized acid to the water-only extractor. It is taught that the amine should preferentially partition into the organic phase and thus substantially not enter into the aqueous phase. '649 specifically teaches that “the acidic materials are extracted into the water as disclosed herein as opposed to merely being scavenged and/or neutralized and allowed to remain in the reaction medium.” To be effective in the above roles, relatively high levels (as high as 10 wt %) of the amine additives are needed. However, such high levels of amines can cause issues with the extractor phase separation, such as generating emulsions and increasing catalytic metal losses.
U.S. Pat. No. 4,567,306 teaches the use of amines to neutralize acidic species to maintain catalyst activity. It does not teach what happens to these amines (other than being volatilized out with the product) and does not teach how to remove the salts so formed. Eventually, the salts will build up until they precipitate.
U.S. Pat. No. 8,110,709 claims the use of amines to trap acidic impurities, then the use of an ion exchange column to remove the resulting amine salts. Similarly, U.S. Pat. No. 7,495,134 teaches the addition of secondary amine additives to precipitate acidic salts, which are removed by filtration.
Some hydrolysis of undesirable phosphorous species is acceptable. U.S. Pat. No. 5,288,918 teaches that it is important to hydrolyze some ligand degradation products that act as catalyst poisons or inhibitors. This can be done without significant hydrolysis of the desirable hydrolyzable ligand by careful control of the effective pH of the system, since these ligand degradation species decompose faster than the desirable ligands in specific pH ranges. U.S. Pat. No. 5,741,944 teaches that the preferred pH range of the acid removal zone is 4.5 to 7.5 and most preferably is between 5.6 and 7.0. If the effective pH is lower than that, hydrolysis of all phosphorous esters occurs; however, if it is higher than that, then the catalyst poison hydrolysis rate is too slow and the catalyst becomes poisoned.
Prior art buffered extractors have been based on metal salts of oxyacids such as NaxHyPO4. The buffer is typically preformed and fed at a concentration of >0.1 mmol/L to a countercurrent extractor where the acids are neutralized and removed under carefully controlled pH conditions. It was presumed in the prior art that the control of the pH in the aqueous buffer phase corresponds to an effective acidity control in the reaction zone. Unfortunately, despite the teachings in WO 2012/064586, a slow buildup of fouling materials based on sodium salts has been observed. Adding amines to water at these concentrations without an oxyacid salt buffer present gives unacceptably high pH values and heavies formation in the reaction fluid. To have sufficient buffer capacity, high levels of amines such as pyridine, trialkylamines, and the like gave unacceptably high aqueous pH values (>9).
Notwithstanding the value of the teachings of the prior art, the search for alternative methods and an even better and more efficient means for stabilizing the rhodium catalyst and organophosphite ligand employed remains an ongoing activity. It would be desirable to have a process to reduce or eliminate highly acidic species in the hydroformylation reaction zone in order to minimize ligand degradation while reducing poisoning phosphite levels without the fouling observed with metal salt buffers.